1. Field of the Invention
The present invention relates to image projection apparatus, and more particularly to image projection apparatus, such as microfilm readers, microfilm reader-printers and overhead projectors, wherein Kohler illumination is used.
2. Description of the Related Art
Image projection apparatus, such as microfilm readers, microfilm reader-printers and overhead projectors, comprise an illumination system for illuminating originals (films), and a projection lens for projecting an image of the illuminated original on an image receiving surface. Such image projection apparatus realize Kohler illumination for concentrating the light from a light source on the pupil position of the projection lens to achieve a higher illumination efficiency.
On the other hand, in the case where the magnification of projection is to be altered according to the size of originals, one projection lens is changed for another projection lens of different focal length if the lenses are single-focus lenses. Alternatively, when the projection lens is a zoom lens, the magnification of projection is variable by zooming. In the former case, Kohler illumination can be accomplished at any magnification of projection by employing projection lenses of different focal lengths which have the same pupil position. In the latter case, the same can be realized insofar as the pupil position remains unchanged despite zooming. However, when the projection lenses are to be so constructed, designing the lens involves a great limitation and therefore entails the problem that the projection lens increases in the number of component lenses and becomes costly.
To fulfill the requirement for Kohler illumination at any magnification, an arrangement is known wherein the condenser lens is movable along the optical axis. The condenser lens is moved along the optical axis manually or automatically, whereby the position for the lens to converge the light from the light source is altered according to the magnification. This arrangement nevertheless requires a mechanism for moving the condenser lens, consequently giving rise to the problem of making the image projection apparatus larger and complex in construction. Additionally, the condenser lens, if manually movable, must be manipulated by the operator and therefore poses a problem as to the ease of handling.